1. Field of the Invention
The present invention relates to a semiconductor memory device and more specifically, relates to an SRAM.
2. Description of the Background Art
According to the SRAM, since it is not necessary to perform refreshing operation in an on state, the SRAM is easy to use but there are many elements in one memory cell and its occupied area is large. Then, reduction in the cell area has been required. According to the conventional SRAM, there is an example of a layout of a cell in which a word line is longer than a bit line, as disclosed in Japanese Laid-open Patent Publication No. 9-270468 (corresponding U.S. Pat. No. 5,744,844) and Japanese Laid-open Patent Publication No. 10-178110 (corresponding U.S. Pat. No. 5,930,163). Thus, speeding up is implemented and a cell area is reduced by reducing the length in the bit line direction.
Furthermore, with respect to miniaturization, a phenomenon of distortion of resist pattern on a wafer (optical proximity effect) becomes quite noticeable because of optical interference in a photolithography machine. In addition, with respect to the process of etching, there is generated pattern distortion after etching caused by a micro loading effect. The micro loading effect is a phenomenon in which as patterns having large difference in density are matched, an etching rate is lowered in a depth direction. In recent years, in order to make such pattern distortion minimum, an optical proximity correction (OPC) technique which previously and automatically corrects a mask pattern in the process of photolithography by a CAD technique has been developed and used.
According to the SRAM, it is necessary to take out data from each memory cell through a bit line at high speed. The data is output from each memory cell through a bit line, an I/O line, a sense amplifier, a read data bus, and an output buffer. At this time, when a signal level output on the bit line from the memory cell is 0.1V, a signal level output on the I/O line through the bit line is about 0.05V, which becomes very small. Therefore, it is likely to receive an influence of noise, and a sense amplifier with high precision is needed in order to amplify it to a Vdd level of about 1.8V.